Porous ceramic membranes are most frequently used to fractionate or concentrate liquid solutions. However, their utility in gas phase separations has not been adequately explored although early ceramic membranes were used to process isotopic forms of volatile uranium hexafluoride for use in nuclear weapons and nuclear fuels. In the past ten years, new ceramic membranes have been developed which have improved permselectivity characteristics. The most useful of these membranes have been fabricated from .gamma.-alumina via the sol-gel process. For example, alumina porous particulate ceramic membranes having typical pore diameters of about 40-50 .ANG. were described by Ulhorn for gaseous separation. However, these existing porous ceramic membranes have been unable to adequately discriminate among gaseous species, except when employed at very high pressures. In general, existing ceramic membranes exhibit only moderate gas phase permselectivities. What is desired is a gas phase separation method that offers improved permselectivity over existing porous ceramic membranes.
Porous ceramic membranes have many properties which make them especially appealing for use in gas phase separations. Of particular interest is their stability under a wide range of conditions including high and low temperatures, oxidizing and reducing atmospheres, and the presence of organic solvents such as benzene, toluene, or hexane. Their stability makes these membranes appropriate for use in a variety of applications which have not previously been feasible because of the limitations of commercially available polymeric membrane systems, which provide a physical barrier through which permeation is thought to occur by adsorption or dissolution, diffusion, and desorption steps.
There are four basic mechanisms by which permselective transport through a porous material can take place. These mechanisms are viscous flow, Knudsen diffusion, molecular sieving and surface diffusion. The relative importance of each mechanism in transport depends upon the surface characteristics of the membrane and on the relative contributions of every other transport mechanism. Viscous flow dominates the transport process when the membrane pore diameters are relatively large, for example greater than 0.1.mu.. A transition from viscous flow to Knudsen diffusion takes place when the mean diameter of the pores approaches 0.1.mu.. The transition from Knudsen diffusion to molecular sieving occurs when molecules larger than the pores are physically excluded from entering the membrane. Thus, the onset of molecular sieving depends upon the size of the gas molecules being transported. This transition typically takes place when the pore diameter approaches 5 .ANG.. At present, however, it is not clear when surface diffusion is an important transport mechanism, nor when transport shifts from a Knudsen mechanism to a surface diffusion driven mechanism.
For particulate ceramic membranes, the transport process is usually viewed in terms of direct passage of molecules through the 10-100 nm void regions constituting the pores which form continuous paths through the membrane. This view of ceramic membranes usually requires that the primary mechanism of gas phase separations be Knudsen diffusion. It should be noted that while the transport occurs via physically real pores in the membrane, these pores are seldom cylindrical in shape, but are usually irregular and often form tortuous paths.
The permeability of a gas transported through a porous membrane by Knudsen diffusion may be expressed as EQU P.sub.m =2.epsilon..mu..sub.k .mu.r/3RTL (1)
where .epsilon. is the porosity of the solid, .mu..sub.k is a shape factor for Knudsen diffusion, r is the mean radius of the pores in the material, R is the gas constant, T is the absolute temperature, L is the thickness of the membrane and .nu. is the mean molecular speed given by EQU .nu.=(8RT/.pi.M).sup.1/2 ( 2)
where M is the molecular weight of the permeating molecule. Consideration of these equations makes it apparent that, since the permeability of a porous ceramic membrane is independent of pressure, the Knudsen diffusivity is also independent of pressure.